Pulse oximetry is a non-invasive medical technique useful for measuring certain vascular conditions, wherein light is passed through a portion of a patient's body which contains arterial blood flow. An optical sensor is used to detect the light which is passed through the body and variations in the detected light at various wave lengths are then used to determine arterial oxygen saturation and/or pulse rates. Oxygen saturation may be calculated using some form of the classical absorption equation known as Beer's law.
Accurate measurements of these and other physiological functions are predicated upon optical sensing in the presence of arterial blood flow. An ear lobe or a finger may conveniently serve for this purpose, since each is an easily accessible body part through which light will readily pass. U.S. Pat. Nos. 4,825,872 and 4,825,879 describe finger sensors which wrap around the finger to thereby permit oximetry measurements to be made. The disadvantage with these devices is that they are either cumbersome to attach to the finger or do not provide solid contact between the finger and the sensor.
U.S. Pat. No. 4,685,464 shows another pulse oximetry sensor using a clothes pin type device. The advantage with this device is that it is very easy to attach to the patient. The disadvantage with this device is that the traditional clothes pin spring mechanism, does not allow the upper and lower halves of the sensor to lie parallel to the upper and lower surfaces of the finger. Rather than clamping the surfaces of the finger with consistent pressure and even physical contact, such devices do not conform to the actual dimensions of the upper and lower surfaces of the finger. The interior angle of the inner surfaces of such clamps are limited by their simple fixed pivot point design to whatever angle results from the thickness of the finger being clamped, rather than the actual angle of the finger's surfaces. As a result, such fixed pivot point, spring clamp mechanisms unevenly concentrate the pressure created by the spring on the points of the finger that happen to contact the inside surfaces of the clamp's upper and lower halves, rather than evenly distributing physical contact and balancing the gripping pressure over the desired contact surfaces of the finger, thereby resulting in localized constriction which decreases the flow of blood and otherwise creates an unnatural or artificial condition.
Clamps for fingers and other body parts are called upon to make critical, highly sensitive and easily disrupted optical and other medical sensory measurements and tests. Proper measurement of physiological functions such as arterial blood flow depends on the ability of a clamp to firmly yet gently grasp the surface of the region being tested so as to make a "quality" contact, thereby permitting accurate optical or electrical/resistive measurements as well as other advanced and conventional sensing techniques. The only way a clothes pin type clamp can obtain the proper physical contact with the surfaces of the finger is if the thickness and angle of the finger surfaces just happen to match the angle of the fixed pivot point jaw and the resultant distances between the clamp's inner surfaces. Otherwise, the only way to obtain sufficient physical contact with the region being tested using a clamp with a fixed pivot point is to increase the pressure on the body part, thereby causing that body part to conform to the predetermined dimensions of the clamp. In short, a clothes pin type clamp not only can result in an inaccurate sensory measurement due to inconsistent surface contact and pressure, but can actually induce an error through the constrictive conditions it creates.
It would be desirable therefore if a clamping device for a sensor could be developed which utilized a hinge mechanism which did not have the disadvantages of the fixed pivot point clamp, and which permitted the upper and lower halves of the sensor clamp to adjust to the dimensions of the body part being sensed so as to lie parallel to the upper and lower surfaces of that body part. Such a device should be capable of gently grasping the surface(s) being sensed with even contact and pressure, despite the infinite combinations of the angle or slope between the surfaces being contacted and the distances between those surfaces (e.g. the thickness of the ear lobe or finger). It is also desirable that the device should conform to the surfaces being contacted, rather than those surface being forced to conform to the dimensions and properties of the clamping device.
Finally, for those instances when increasing the physical pressure on the body part being evaluated is desired or even required as part of the test procedure, the clamping device should have inner surfaces that will adjustably conform to the surface being tested while applying increased yet evenly distributed pressure, without forcing or distorting the body part being tested to conform to the inner surfaces of that clamping device.